An active matrix substrate used in liquid crystal display devices and the like has a switching element such as a thin-film transistor (hereinafter, “TFT”) in each pixel. Conventionally, a TFT with an amorphous silicon film as an active layer (hereinafter, “amorphous silicon TFT”) or a TFT with a polycrystalline silicon film as an active layer (hereinafter, “polycrystalline silicon TFT”) has been widely used as such a switching element.
The use of an oxide semiconductor as the material of the TFT active layer, instead of amorphous silicon or polycrystalline silicon, has been recently proposed. Such a TFT is referred to as an “oxide semiconductor TFT.” Oxide semiconductors have a higher mobility than amorphous silicon. Therefore, the oxide semiconductor TFT can operate at a faster speed than the amorphous silicon TFT. Furthermore, the oxide semiconductor film is formed with a process that is simpler than for the polycrystalline silicon film, and thus, the oxide semiconductor film can be applied to devices requiring a large area.
An oxide semiconductor TFT with a top-gate structure is disclosed in Patent Document 1. The oxide semiconductor TFT disclosed in Patent Document 1 undergoes plasma treatment to form a source/drain region on the oxide semiconductor layer thereof.